다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

13. 벼잎선충의 약제처리 방법별 방제 효과

Random distributed feedback fibre lasers

Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-006451.pdf

Photon Enhanced Thermionic Emission for Solar Concentrator Systems

Smooth wavelength tuning over 10 nm has been realized in a dispersion-tuned harmonically mode-locked fiber ring laser. Supermode noise is suppressed by 13 dB by adding a semiconductor optical amplifier (SOA) into the cavity and the suppression improves as the SOA gain increases. 5.3-ps pulses are obtained at a repetition rate of 10 GHz. A simple numerical model demonstrates the effects of the intracavity dispersion and the SOA on the pulse characteristics.

A stable smoothly wavelength-tunable picosecond pulse generator

A general framework based on the fluctuation-dissipation theorem has been outlined for the study of the spontaneous thermal fluctuations in optical fibers. The goal is to seek a unified scheme to analyze the two types of intrinsic noises found in fiber lasers and interferometric fiber-optic sensors, namely, the thermoconductive noise and the thermomechanical noise. Some outstanding questions in the current theories are addressed. These include: (a) the underlying relation between the thermoconductive and the thermomechanical noises, (b) the lack of a fully disclosed theory for the thermoconductive noise in passive fibers, and (c) the low-frequency restriction in the current theory of the thermomechanical noise. Specific analyses based on the proposed approach find excellent agreement with existing theories.

General treatment of the thermal noises in optical fibers

The spontaneous length fluctuation of optical fibres caused by mechanical dissipation is analysed using a one-dimensional model based on the Fluctuation-Dissipation Theorem. Scale estimate shows evidence that the 1/f thermal noise originated from this fluctuation dominates at low frequencies in fibre interferometers and fibre cavities.

Intrinsic thermal noise of optical fibres due to mechanical dissipation

Optical sampling by cavity tuning (OSCAT) enables cost-effective realization of fast tunable optical delay using a single femtosecond laser. We report here a dynamic model of OSCAT, taking into account the continuous modulation of laser repetition rates. This allows us to evaluate the delay scan depth under high interferometer imbalance and high scan rates, which cannot be described by the previous static model. We also report the demonstration of remote motion tracking based on fast OSCAT. Target vibration as small as 15 µm peak to peak and as fast as 50 Hz along line-of-sight has been successfully detected at an equivalent free-space distance of more than 2 km.

Dynamic optical sampling by cavity tuning and its application in lidar.

The fundamental thermal noise in fiber-optic sensors remains to be an interesting research topic. In particular, its spectral behavior in the infrasonic frequency range has yet to be observed experimentally. In this paper, we assess the feasibility of probing the thermal noise floor at infrasonic frequencies with two sensor configurations: 1) fiber Fabry-Perot strain sensors and 2) Mach-Zehnder–Fabry-Perot hybrid phase sensors. In each case, we compare the theory-predicted thermomechanical noise (the dominant thermal noise in fibers at low frequencies) with other potential system noises such as laser noises and detector noises. Our analysis indicates that the thermal-noise-limited fiber-optic sensing can be very difficult to achieve with the strain-sensing scheme, but much more feasible with the hybrid phase sensors.

Lingze Duan

Papers

A stable smoothly wavelength-tunable picosecond pulse generator

General treatment of the thermal noises in optical fibers

Intrinsic thermal noise of optical fibres due to mechanical dissipation

Dynamic optical sampling by cavity tuning and its application in lidar.

Thermal Noise-Limited Fiber-Optic Sensing at Infrasonic Frequencies